The present invention relates to apparatus and a method for imperceptibly sending digital information during the active portion of an interlaced video signal and in particular to a method of sending such digital information by modulating a Fukinuki carrier with the digital data using quadrature amplitude modulation (QAM) techniques.
Currently there are several systems which send limited amounts of digital information in a television signal. These include teletext, in which information in a magazine format is sent with a television signal, and closed captioning for the deaf, in which subtitles for television programs are transmitted as a part of the television signal. One method of transmitting this information is to place standard encoded digital data into the television signal during the vertical blanking interval of each field. Another method transmits the digital data by modulating a separate carrier in the audio portion of the television signal. Using these methods, a limited amount of digital information may be transmitted with the television signal in a manner which does not affect the quality of the image or its accompanying sound as perceived by a viewer.
Many other methods are known for transmitting digital information through an analog channel. For example, frequency shift key (FSK), phase shift key (PSK) and quadrature amplitude modulation (QAM). All of these methods are described in a textbook by M. Schwartz entitled Information Transmission, Modulation and Noise, McGraw Hill, 1990. None of these methods can be used in their conventional sense to transmit digital information in the active (i.e. image conveying) portion of a television signal, however, since they would perceptibly interfere with at least some portion of the reproduced image.